1. Field of the Invention
The present invention relates to an inkjet printer.
2. Discussion of the Background
As shown in FIG. 4, an inkjet printer has a structure for ejecting ink droplets from nozzles 12 aligned on the lower surface of a printing head 14 and landing the ink droplets on the surface of a medium 30 mounted on a platen 20 in a dot pattern. Further, the printer has a structure for printing pictorial diagrams and characters formed by arrangement of plural ink dots on the surface of the medium 30.
In this inkjet printer, there is a drawback that, when the ink droplets are landed on the surface of the medium 30 in a dot pattern, the ink droplets permeate the medium 30 broadly in the surrounding parts of the landed points, and the pictorial diagrams and characters formed by arrangement of plural ink dots printed on the surface of the medium 30 become smeared and blurred.
As an inkjet printer that can solve such drawback, a printer as disclosed in Publication of Japanese Patent Application No. Sho-62-144955 or Publication of German Patent No. DE10056703C2 has been proposed.
In this printer, as shown in FIG. 4, a preheater 40 for heating a medium 30 before printing in advance, and an after-heater 50 for heating the medium 30 immediately after printing are provided. Droplets are prevented from permeating broadly in the surrounding parts of landed points by having preheated the medium 30 with the preheater 40 for warming it before printing so that the droplets landed on the warmed surface of the medium 30 may be dried early. Further, the droplets are prevented from permeating broadly in the surrounding parts of landed points on the medium 30 by early drying the droplets adhering to the landed points on the medium 30 immediately after printing in partly undried conditions with the after-heater 50. Thereby, the pictorial diagrams and characters formed by arrangement of plural ink dots without smearing are printed sharply and clearly on the surface of the medium 30.
By the way, in the conventional inkjet printers as disclosed in these publications, heating means (not shown) for heating the medium 30 to be carried onto a central portion 22 of the platen below the traveling path of the printing head 14, on which the ink droplets ejected from the nozzles 12 of the printing head landed, is not provided.
The reason is as follows: in the case where such heating means is provided, by the heating means, it is possible that the medium 30 to be carried onto the central portion 22 of the platen below the traveling path of the printing head 14 is heated to dry the ink droplets landed on the surface of the medium 30 early. However, simultaneously, by the heating means, the printing head 14 traveling above the central portion 22 of the platen is also heated. Then, by the influence of heat from the heating means, the ink supplied to the printing head 14 is dried, the ink is solidified within the nozzles 12 having thin diameters of the printing head, and the nozzles 12 are clogged.
By the way, the conventional inkjet printer mainly uses general-purpose water-soluble ink or lactate ink as ink supplied to the printing head 14. However, such water-soluble ink and lactate ink do not have sufficient water resistance or weather resistance. On this account, a printer using such water-soluble ink and lactate ink is not suitable for printing pictorial diagrams and characters on a medium 30 for outdoor display advertisement or the like.
On the other hand, solvent ink consisting primarily of an organic solvent has sufficient water resistance and weather resistance. Accordingly, the solvent ink as above is suitable for printing pictorial diagrams and characters on a medium 30 for outdoor display advertisement or the like. However, the solvent ink has extremely high permeability to the medium 30, and, when ink droplets thereof are landed on the surface of the medium 30, the solvent ink of the ink droplets permeate the medium 30 early and broadly in the surrounding parts of the landed points and disappear. On this account, dots of the ink can not be clearly fixed at the landed points on the surface of the medium 30, and pictorial diagrams and characters formed by arrangement of plural ink dots with no smear can not be printed on the surface of the medium 30 clearly.
Note that, as the medium 30, a medium 30 coated with various kinds of coating agents on the surface thereof for preventing the ink droplets landed on the surface from permeating the medium 30 in the surrounding parts of landed points and fixing them at the landed points to form dots having small diameters has been developed. However, the medium 30 coated with such coating agents is expensive and it can not be used as a medium 30 that is generally and widely used.
Further, as a medium 30 for outdoor display advertisement or the like, both materials such as vinyl chloride films that are easily softened by being heated at low temperature and polyester films that are not easily softened even by being heated at high temperature are used.
On the other hand, as disclosed in the above publications, in the conventional inkjet printer, the temperature at which the medium 30 is heated by the preheater 40 and the after-heater 50 is set unadjustably at constant and relatively high temperature. Accordingly, if the medium 30 carried on the platen 20 of the inkjet printer is a vinyl chloride film or the like, the medium 30 will be overheated at high temperature and softened by the preheater 40 and the after-heater 50, and thereby, the medium 30 can not be carried on the platen 20 smoothly.